Listened to most of the Jeff Kuhner Show and now Howie Carr, and they both are discussing this story as if it is completely factual, all statements attributed to Bannon and Trump’s response.
Was hoping it would prove to be fake news, but nothing to refute it so far.
There are two statements in issue: Bannon's alleged statement that Trump Jr.'s meeting with the Russians was "treasonous," and Trump's statement that Bannon "lost his mind."
The first quote (Bannon's) is dubious. The only source for that is a British newspaper which says it is in a forthcoming book which quotes Bannon. So double hearsay at best. (The newspaper may be misquoting the book, and/or the book may be misquoting Bannon.) On the other hand, I haven't seen Bannon disavow the quote.
The second quote (Trump saying that Bannon "lost his mind") is real; it's been confirmed by the White House. So Trump apparently believes that Bannon said what he's alleged to have said.